The problems associated with the separation of suspended solids from industrial process waters are well known to those skilled in the art. For example, see the background of the invention section of U.S. Pat. No. 3,601,039.
One such means of enhancing the settling or filtration of suspended solids from aqueous media is by chemical treatment. In particular, the chemical treatment process used to enhance the dewatering of activated sludge, which is typically anionic in nature, is described by the addition of a cationic flocculant to the sludge while agitating the sludge. The flocs that form are then subjected to a dewatering operation. This treatment process consists of neutralizing the negative electric charge of the sludge by the addition of a positively charged flocculent, the subsequent formation of small flocs, followed by the agglomeration of the small flocs into larger flocs that settle out of solution prior to the dewatering operation. For example, the use of polyamines as cationic flocculants for thickening and dewatering operations is taught in U.S. Pat. No. 3,975,266.
One objective of improving the flocculation process is to attain better dewatering performance by the formation of larger, sturdier flocs. U.S. Pat. No. 3,235,490 teaches the use of various chemical additives including certain polymers of acrylamide cross-linked with N,N′-methylenebisacrylamide. The polymers are claimed to flocculate the suspended solids in an aqueous media by capture of fine particles within the structured polymer network. However, '490 teaches that said copolymers are only effective as cationic flocculants under acidic or neutral conditions.
U.S. Pat. No. 3,698,037 teaches a method of releasing water from activated sludge utilizing cross-linked, cationic emulsion copolymers.
U.S. Pat. No. 4,705,640 teaches that the flocculation performance of certain cross-linked, cationic acrylamide dispersion copolymers is improved by subjecting the polymer solution to mechanical shear prior to application.
U.S. Pat. No. 6,294,622 B1 teaches the process for the preparation of certain cross-linked, cationic acrylamide emulsion copolymers that exhibit improved flocculation performance without the use of mechanical shear. The process described for preparing the cationic acrylamide copolymers comprises polymerizing the monomers while adding the cross-linking agent (chain-branching agent) continuously to the mixture throughout the course of the reaction. Furthermore, '622 teaches that said addition of the cross-linking agent must begin before about 50%, preferably about 25% of the monomer conversion is achieved.
The prior processes as exemplified by U.S. Pat. No. 6,294,622, exhibit the potential to produce a polymer with a higher degree of cross-linking resulting from the continuous addition of a cross-linking agent over the extent of the entire polymerization reaction.
Higher cross-linking may result in the reduction in the net cationic charge available for neutralization of the negative charge on the substrate by trapping or shielding a portion of the charge within the three-dimensional polymer network. Moreover, the process shows a further shortcoming in that, although the polymers formed possess a larger molecular weight relative to their linear counterparts, their net size, or measure of true hydrodynamic volume, is compromised. The interlocking nature of the extensively bound polymer network prevents full dispersion or extension into solution, particularly within the limited time of the application.
WO 02/02662 A1 is directed to structurally modified water-soluble polymers made via polymerization of an aqueous solution of monomers under free radical polymerization conditions. At least one structural modifier is added to the polymer solution after at least 30% polymerization of the monomers has occurred. The so-called structural modifiers can comprise either a chain transfer agent and/or a cross-linking agent. As is apparent from Ex. 7 of this publication, when a cross-linking agent is added alone in the later stages of the reaction, as the only structural modifier, it is always shot fed (i.e., added all at once in a single treatment). However, as shown in Tables 29 and 25, when the cross-linking agent is fed over time, it is always fed in combination with a chain transfer agent (i.e., “co-fed” with a molecular weight modifying agent).